This invention relates to cable television (CATV) and wireless (MMDS or VHF/UHF) transmission systems. More particularly, the invention relates to a system and method for doubling the number of audio channels to be transmitted over the RF bandwidth typically reserved for a single audio transmission.
With the increasing array of services from CATV and wireless network operators, it has become imperative for operators to offer more services in the same amount of RF transmission bandwidth. Moreover, wireless pay television systems, whether MMDS or conventional VHF/UHF television, are generally constrained to far fewer channels than the conventional CATV systems with which they compete. The challenge is offering more channels to subscribers within the spectrum constraints imposed by government regulations.
For a typical television program, since the video portion of the program occupies most of the available 6 MHz on an NTSC television channel, much of the research toward maximizing the amount of bandwidth has been traditionally devoted towards compressing and minimizing the amount of bandwidth the video information occupies. However, it is equally important to minimize the amount of bandwidth which is dedicated to the audio portion of the program, or maximize the amount of audio channels which can be transmitted within the bandwidth typically reserved for a single audio channel. Accordingly, the present invention is directed towards maximizing the amount of information transmitted within the audio portion of the RF bandwidth.
There are several audio transmission systems which have attempted to increase the number of audio channels transmitted within the bandwidth typically reserved for a single audio channel. For example, the German Zweiton audio transmission method utilizes two RF audio carriers to provide either a selection between two language audio options, or a single stereo audio derived from the two carriers. Using this system, the audio is restricted to a single transmitted video program.
Another method for transmitting audio is the xe2x80x9cFM-Squaredxe2x80x9d method for transmitting audio in analog satellite transmissions. The FM-Squared method uses one or more FM audio carriers added to the frequency spectrum above the baseband video signal to create a composite video signal. However, the FM-Squared method is similarly limited to the German Zweiton audio transmission method since alternative audio transmission formats are not permitted on the two FM audio carriers.
Accordingly, there exists a need for providing more channel capacity within the same amount of transmission bandwidth while maintaining the quality of the transmitted audio and providing flexibility regarding the audio format.
The present invention provides for the transmission of two audio services, each related to a different video program, within the bandwidth typically reserved for a single audio channel. Each audio service may be processed as a mono, stereo (BTSC method) or encrypted signal prior to modulation onto its own audio carrier. The two audio carriers are summed together and applied to an RF modulator. The frequencies of the audio carriers are selected to ensure that the quality of the video program is not degraded, the video channel mask bandwidth is not exceeded and interference from the unselected audio carrier is negligible. An alternative method of transmitting two audio services on single audio carrier is also presented whereby the second audio service is modulated onto a subcarrier of the main audio carrier of the first audio service.
Accordingly, it is an object of the present invention to transmit two audio services, each related to a different video program, within the RF bandwidth typically reserved for a single audio service without exceeding the video channel mask bandwidth or degrading the quality of the video program.
Other objects and advantages of the present invention will become apparent after reading the detailed description of presently preferred embodiment.